drm/mga200g: Hold a proper reference for cursor_set

Looking up an obj, immediate dropping the acquired reference and then
continuing to use it isn't how this is supposed to work. Fix this by
holding a reference for the entire function.

While at it stop grabbing dev->struct_mutex, it doesn't protect
anything here.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>